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By Terry Mattioli, Buck Bradshaw, Terry Fagg, Larry G. Nisle

Deposition of calcium carbonate scale on activated carbon in the Pinson Mining mill carbon column circuit was causing erratic gold recovery rate, long acid wash time, high carbon attrition and long strip times. Continuous addition of a blended antiscalant into the mill thickener overflow at 8 ppm reduced the average carbon acid wash weight loss from approximately 10% to < 2%. Cleaner carbon improved the gold loading rate in the mill column. The loading on Column I was increased to 175-225 oz/ton carbon from 125-150 oz/ton for a two-day carbon advance cycle. The higher loading capability has given Pinson more flexibility and better control of gold flow through their carbon adsorption, stripping and electrowin­ning processes. Preventing calcium carbonate build-up on activated carbon with this antiscalant reduced acid wash batch time. This enabled Pinson to acid wash and regenerate every batch of stripped carbon for all the adsorption circuits. Shorter strip times have also resulted from the lowering of the calcium carbonate scale level in the loaded carbon. Carbon consumption has been reduced and is now averaging 0.046 lb/ton ore for the first six months of 1990 compared to 0.072 and 0.083 lb/ton respectively in 1989 and 1988.

Jan 1, 1990

By Jim McMaster, Charles W. Kenney

The role of oxygen in cyanide leaching ia well understood in the gold industry. Oxygen enrichment has taken on a real, practical meaning to operators with the were to evaluate the effect of oxygen enrichment on the kinetics of heap leaching gold ores, to examine methods of oxygen application in the heap, and to develop preliminary economics for the process. Several series of column tests were completed increasing preaeration, applications. use of pure oxygen for leaching, and in CIL in the test program. Oxygen enrichment may have a significant place in heap leaching as well. In preliminary column tests sponsored by Kamyr at Hazen Research, Inc. , oxygen-purged columns showed an increase in both the leaching rate and maximum recovery of gold and a reduction in cyanide consumption compared to columns run with air. Potential benefits of using oxygen in heap leaching include faster leaching kinetics, shorter cycle time, higher recoveries, and more efficient use of permanent or reusable pad space. The concept requires confirmation of technical and economic feasibility on a commercial scale.

Jan 1, 1990

By Peter A. Spencer

Much criticism has been levelled at bacterial oxidation over the quantities of air required for successful operation. Examination of the oxidation reactions and the assumptions concerning oxygen transfer made in the literature (1) indicate that the aeration requirements can be much less than those beirig suggested. Observations made on a large scale pilot test have confirmed reasonable aeration levels in extended site operation over a six month period. These aeration levels have implications for the engineering design and the power needed for aeration and agitation. Thus, these parameters effect both capital and operating costs as well as influencing the complexity of the design and normal operation. A number of companies have developed bacterial oxidation cultures intended for commercial ·expl oi tat.ion ( 2, 3, 4). Most of these cultures have been based on Thiobacillus and operate at temperatures between 30°c and 40°c. This temperature range limits their application in hotter regions unless means for both heating and cooling are provided in the design. Thiobacil!us cultures are also reported as not able to oxidize arsenic (III), which may limit the application of these cultures to successfully treat arsenopyritic ores and concentrates (5). BacTech (Australia) Pty Ltd has developed a culture which operates at temperatures above 4o0 c and which has been tested in a large site based continuous pilot plant at up to 55°c over a six month period. The culture showed resistance to high levels of soluble arsenic, allowing the arsenic to be retained in solution for control led precipitation and disposal. Data for. the arsenic precipitates indicated that these waste solids were relatively stable and, under the correct circumstances, were suitable for disposal into tailings dams without causing environmental problems.

Jan 1, 1990

By Roger Nendick

The recent developments in refractory precious metal ore processing (Bio-oxidation, Autoclaving), and the revitalization of the more traditional roasting processes have attracted great attention in recent years and are indeed the focus of todays symposium. It must always be remembered, however, that the term refractory ore by definition means that the material 'must be capable of being treated at a profit". Given the high cost of the refractory processes, the requirement to be treated at a profit requires very close scrutiny during the evaluation phase of the property development. The M.G.M. film company has the latin motto Ars Gratia Artis meaning Art for Arts Sake. Ingeniatore Gratia Ingeniatoris (a very crude attempt at saying Engineering for Engineerings Sake) should not be the motto for mining companies intent on exploiting refractory gold deposits, but rather the wittily tilted heading for this paper "Take the money and run like a thief". The following hypothetical case studies show that the high technology solution is not always the best financial solution.

Jan 1, 1990

By Dale C. Matthews

The Getchell Gold Hine is located about 40 miles northeast of Winnemucca, Nevada. Gold was discovered at the mine in 1934 and production was started in 1938 and ended in 1967 for its first phase of operation. Gold production during this period was about 800,000 ounces. In 1983, FirstHiss Gold Inc. purchased the property and initiated an exploration program that proved minable reserves of about 1,300,000 ounces of gold remained in the mine. During the exploration period, metallurgical testing was conducted to determine the most economical process that could be utilized for recovery of gold from the very refractory ore. After extensive investigation, pressure oxidation of the ore was selected, Engineering for the new plant commenced in 1987 and construction was completed in 1989 at a cost of about $87,400,000.00, not including financing cost.

Jan 1, 1990

By George Hope, Edward H. Smith

In late 1989, LAC Minerals purchased 65% of Bond International Gold and as a result became controlling shareholder of the El Indio Mine through Bond and San Jose. El Indio is a world class, complex sulphide gold deposit located in the hydrothermal alteration belt of the Andes mountains in the 4th Region of Chile. The mine is located in the high Andes about 185 km east of La Serena at an elevation of 4000 meters. The 1450 workers at El Indio live in the mine camp at 3200 m and on days off are bussed to La Serena. The El Indio mine was started in 1979 by St. Joe Minerals Corp., bought by Bond in 1987 and subsequently by LAC in 1989. The mine has produced 2.7 million oz. of gold, 9. 5 million oz. of silver and 170,000 t of copper since start of operation. The mill is currently operating at a rate of 2850 t/d and producing a copper/gold concentrate which is roasted to produce a copper/gold calcine and high quality, saleable crude As203• A small proportion of the precious metals are also produced as bullion from the leach circuit. Total production during 1989 was 224,197 ounces of gold, 1,309,342 ounces of silver and 26,968 tonnes of copper. The average cash cost of production for the period was U.S. $96 per ounce of gold, net of by-product credits. The average realized price was $5.30 per ounce of silver and $1.14 per pound of copper.

Jan 1, 1990

By Robert M. St. Louis, Frank D. Wicks, John E. Huffman, Edward E. Maurer

The treatment of Mercur's refractory ores has been well published; however, the processing of the remaining semi-refractory or oxide ores has not been discussed. Although ore control at Mercur is efficient, refractory vestiges are left with the mill-feed, oxide ores. Also, these ores as mined are not necessarily completely oxidized at Mercur and should be termed semi-refractory ores. This paper will discuss those steps taken at Mercur to enhance the recovery of gold from partially oxidized ores at Mercur. The Mercur mill is a conventional SAG/ball mill grinding circuit feeding a GIL plant. The circuit has been well published and will not be further reviewed.

Jan 1, 1990

By Andrew Briggs, Denis Kidby

VITROKELER cyanide recovery technology has been successfully piloted at the Gabanintha and Cosmo-Howley mines by Nedpac Engineering Pty. Ltd. The pilot campaigns were designed to confirm technical and economic feasibility of the recovery process including the production of base metal concentrates and return of re-useable cyanide to the leach circuit. The capital and operating costs of full-scale plants is now able to be estimated to within ±20% on the basis of hydrometallurgical data. Firm quotes are subject to on-site test-work, and any special requirements or site peculiarities.

Jan 1, 1990

By Peter Greenhalgh, W. Baguley, R. P. Riley

The traditional impeller system for aeration and suspension of slurries, e.g., in bacterial oxidation plants, is the vertical flat blade turbine (VFBT) . This type of impeller is energy-intensive, which results in a heavy-duty design of shaft, gearbox and drive. The VELMIX bio-reactor development programme is designed to evaluate the use of the new generation of hydrofoil impellers which consume less power and thus have poten­tial for capital and operating cost savings as well as being possibly less damaging to the bacteria. The early tests on pyrite oxidation in a 300 litre laboratory pilot tank using the Thiobacillus ferro oxidans bacteria, showed that hydrofoil impellers had superior charc­teristics to the VFBT impeller and to air­ agitated pachucas, with lower energy input. The geometry of the sparge _ring was found to be very important in optimizing the system.

Jan 1, 1990

By Kevin A. Foo, Joseph L. Young, William W. Reid

The title of this paper is "Designing Very Large Bio-oxidation Plants". To our knowledge, only one such plant has been designed and built to date, and that is U.S. Gold's Tonkin Springs plant in Nevada. This plant was commissioned in Spring of 1990 and operated for a short time successfully as a bio-leach plant but U.S. Gold did not have the financial strength to overcome the consequences of construction delays and increased capital cost. The plant has temporarily been 1 put on a standby basis. Recently, < > a potential joint venture between Tonkin Springs Gold Mining Company and Homestake Mining was announced. Homestake will operate the mine with the technology and subsequent improvements would be co-owned by both companies. This paper is not a "how to" type publication because we feel that such an approach could be dangerous if used improperly. Rather, we discuss some of the important aspects of process selection, testing and design that need attention, and some of the lessons learned to date. As with all developments, improvements and new knowledge will accrue as more companies use bio-oxidation technology. The Tonkin Springs plant is fair evidence that bio-oxidation has "arrived" as a technology, and that successful design and operation of plants treating greater than 1000 t/d of feed is possible.

Jan 1, 1990

By Dieter Englehardt

This paper describes the metallurgical evolution of a sulphide flotation plant. Beginning with the original plant, based on laboratory and pilot scale tests performed on initial drill hole composites, it describes the metallurgical changes implemented due to the changing mineralogy of the ore.

Jan 1, 1990

By A. Russell, M. N. Hughes, Jack Barrett

The bacterial oxidation of arsenopyrite has been shown to be an efficient, economic and environmentally acceptable method for the liberation of any gold content . The mechanism of the process is not certain but is usually considered to involve direct and indirect reactions with respect to bacterial involvement. The direct reactions are those in which the bacteria attached to the mineral surface cause its dissolution, with dioxygen being the oxidant. The iron(III) produced in solution is the chemical oxidant in the so-called indirect reactions. Barrett et al 2 showed that a synthetic bio-oxidation solution (with no cells present) could dissolve up to 15% of an arsenopyrite sample, the products of the chemical oxidation being iron(II) and arsenic(III). This paper describes an extension of the investigation of the chemical process and its imp! ication for the mechanism of the bio-oxidation of arsenopyrite.

Jan 1, 1990

By Kenneth B. DeGraaf

The development of precious metal recovery processes using chloride systems has mostly evolved from the development of chloride based treatments for base metal concentrates or ores. The recovery of silver from chloride solutions within a process designed for the treatment of base metals usually is part of a solution purification stage. This paper discusses the development of the silver recovery stage used in the GCM HydroCopper process.

Jan 1, 1990

By John E. Litz, Charles W. Kenney, Wayne Carter

Refractory gold ore pretreatment processes continue to receive much attention. This paper compares the relative economics of treating ores by oxygen pressure oxidation, roasting and bacterial oxidation with or without preconcentration by froth flotation. Capital and operating cost estimates for each method of pretreatment were prepared and these estimated costs and ore sulfide grades then are used to compare the process economics. The study indicates that the most significant item affecting the economics is the amenability of the ore to flotation. The cost benefits of flotation are $10-13 per tonne ore processed (2%-4% s=, 10:1 ratio of concentration). In most cases bacterial oxidation has the lowest overall cost, especially if less than complete oxidation of the sulfides will give acceptable recovery, however bacterial oxidation has yet to be practiced on a large scale. In all situations pressure oxidation shows the highest cost. However, the cost differences between the treatments are small, less than $7 per tonne.

Jan 1, 1990

By Leslie C. Thompson

The heap leach process used for recovering gold from oxide and sulfide ores is operated as a closed circuit system in which the cyanide process solutions are continuously recy­cled. In a well-designed operation there is little or no dis­charge of cyanide to the environment. Spent leached ore residues are water washed to remove most of the trace cyan­ides and precious metals remaining after completion of cyan­ide leaching. After the washing step, small amounts of cyanide, metal-cyanide complexes and nitrates remain in the residue. Most of the cyanide and nitrate residue constituents exist in the pore water of the spent ore rather than in the solid mineral fraction. More than 90% of the complexed cyanides in a freshly washed ore residue are present as free cyanide or weakly complexed metal-cyanides. Natural volatilization and decom­position rapidly remove most of these easily dissociable cyanides from the spent ore. Only the stable, strongly com­plexed metal-cyanides, such as gold, cobalt or the ferrocyan­ides and other constituents such as nitrates have a long term persistence in the spent ore residue (Towill et al). Despite washing and natural removal mechanisms, spent ore has the potential to act as a point source of com­plexed cyanides and nitrates for soil and groundwater con­tamination if inadequately contained (Huiatt et al). The groundwater contamination is caused by the leaching of the soluble cyanide compounds and nitrates from the pore water of the residue.

Jan 1, 1990

By Trent Temple, Curtis Johnson, John Mudge

Newmont Gold Company uses centralized carbon handling facilities to strip loaded carbon from five mills and four leach facilities located along a 38 mile stretch of the Carlin Trend in Nevada. Both Zadra and AARL elution processes are employed at this centralized facility which is located near the company's Gold Quarry operations. The Zadra elution circuit was commissioned in 1985 and later expanded in 1987. This circuit can strip carbon at a rate of 24 tons per day. The AARL elution circuit, commissioned in 1988, also has the capacity to strip 24 tons of carbon per day. Two horizontal, rotating, regeneration kilns are used to reactivate approximately 60% of the stripped carbon before it is returned to the mills or leach facilities. One kiln is located in the Zadra plant, while the other is located in the AARL plant.

Jan 1, 1990

By E. Haug, Quamrul Ahsan, R. Norcross

Cyanide leaching is the most common process employed in the majority of the gold extraction plants. These gold extraction plants naturally generate tailings and/or effluents that contain highly toxic cyanides, In geographic locations with excess precipitation and therefore net positive water balance, a portion of these tailings must be discharged. Before these highly toxic cyanide-containing effluents can be discharged to the waterways, cyanide and any other toxic substance must be destroyed. Geographic locations with high evaporation and low precipitation, as in the western USA, result in net negative water balance and therefore these plants can operate with a zero discharge option. However, with the current stringent environ­mental regulations and concerns for bird and wildlife mortality, even these operations may be required to partially treat their tailings to preclude potential harm to wildlife. Furthermore, heap leach opera­tions are now generally required to detoxify the heaps before abandonment. It is, there­fore, apparent that all gold extraction plants may be required to install some form of effluent treatment in order to achieve the above objectives,.

Jan 1, 1990

By P. G. Semple, K. W. Major, P. Eng.

The recent increase in development of refractory gold deposits has resulted in changes to the traditional treatment processes. The selection of the preferred refractory treatment process is dependent on the mineralogy of the ore. One process, which has recently gained increasing attention, utilizes dry grinding and roasting of the whole ore. This paper will discuss some of the considerations to be addressed during the design of the process fl owsheet and what the effects of the various alternatives will be on the plant capital and operating costs. The paper will be based, in part, on current experiences with dry grinding, roasting plant design and start-up.

Jan 1, 1990

By Joe D. Allison, Tom D. Baugh

The analysis for total cyanides (CN, or the cyanide due to the combination of free, weakly associated and refractory cyanide complexes) is required by most regulatory agencies for determination of effluent compliance. This method requires a distillation of the sample at low pH's and is most often sent to contract laboratories at a high time and monetary cost. This paper describes a new micro­distillation apparatus which allows for a multiple of CN,"' analyses to be carried out simultaneously saving both time and money. It can be used in the field, since only electrical power is required for the method. While not being appropriate for reporting values to the respective regulatory agencies as of yet, it is quite valuable for screening a large number of samples prior to submission of the samples to a contract lab for "official" reporting.

Jan 1, 1990

By S. K. Bhatt

This paper reviews major developments in continuous haulage technology for underground coal mines. Haulage systems, both in use and development, are investigated through a comprehensive literature search and visits to mines and manufacturers. The systems include flexible conveyor train, mobile conveyor, multiple-unit continuous haulage, extensible, and belt turning systems. Strengths and weaknesses of the systems are assessed in light of their operating under remote control and their application to the computer-assisted continuous mining machine being developed by the U.S. Bureau of mines. Through mining scenarios, candidate haulage systems are conceptualized to be consistent with the capabilities of the mining machine.

Jan 1, 1990